Transmission system with improved tone detection

Abstract

In a transmission system a pair of signalling tone signals is transmitted by a transmitter (2) to a receiver (6) via a transmission channel (4). In the terminal (6) a tone detector is used to detect the presence of the pair of signalling tone signals. In order to improve the reliability of the tone detector in the case of differing strength of both tone signals, the decision on the presence of the pair of signalling tone signals is based on a combined strength measure which is determined by an adder (23). The combined strength measure is compared with a reference level TH 2 . In a further embodiment, also a check on the level of the individual tone signals is done with comparators (11) and (9), in order to specify the maximum strength difference tolerated.

I claim: 
     
       1. A transmission system comprising a terminal and a transmitter for transmitting simultaneously a plurality of tone signals via a transmission medium to the terminal, said terminal comprising: strength measuring means for deriving tone strength measures for said plurality of tone signals; and   decision means for deciding on the presence of a plurality of tone signals if a combined strength measure of the plurality of tone signals exceeds a first threshold value, and individual strength measures of the tone signals involved exceed a second threshold value, said first threshold value being greater than the product of the number of tones in said plurality times the second threshold.   
     
     
       2. A terminal for receiving a simultaneously transmitted plurality of tone signals, said terminal comprising: strength measuring means for deriving tone strength measures for said plurality of tone signals; and   decision means for deciding on the presence of a plurality of tone signals if a combined strength measure of the plurality of tone signals exceeds a first threshold value, and individual strength measures of the tone signals involved exceed a second threshold value, said first threshold value being greater than the product of the number of tones in said plurality times the second threshold.   
     
     
       3. A tone detector for detecting the presence of a plurality of tone signals in an input signal, said tone detector comprising: strength measuring means for deriving tone strength measures for said plurality of tone signals; and   decision means for deciding on the presence of a plurality of tone signals if a combined strength measure of the plurality of tone signals exceeds a first threshold value, and individual strength measures of the tone signals involved exceed a second threshold value, said first threshold value being greater than the product of the number of tones in said plurality times the second threshold.   
     
     
       4. A method for receiving a simultaneously transmitted plurality of tone signals, said method comprising: deriving a tone strength measure for each one of said plurality of tone signals; and   deciding on the presence of a plurality of tone signals if a combined strength measure of the plurality of tone signals exceeds a first threshold value, and individual strength measures of the tone signals involved exceed a second threshold value, said first threshold value being greater than the product of the number of tones in said plurality times the second threshold.